Idk why I’m being so mean to my mum

So my grandma died a year and a half ago, and my mum has been reading buddhist scriptures every day for 2 hours and burning paper for her ever since. She cries whenever she brings her up and doesn’t seem to be moving on.

I feel like I’m being an absolute bitch to her because I am someone who very much doesn’t believe in the afterlife etc. (she wasn’t religious before her mum died either) so whenever she talks about her mum “visiting in dreams” or when she puts food on the mini alter she set up for my grandma to “eat” I always cringe and I can’t hide it. Furthermore she always forgets about the food and won’t let anyone touch it until she says so it ends up going to waste and getting thrown away after.

Today I accidentally lost it at her because she made me fold the paper she was going to burn with her for my grandma and I said “grandma is dead she’s not ‘receiving’ anything”. She started crying after I went upstairs and I feel awful :/ how do I help her move on? She’s been doing this for a year now and it doesn’t seem to be helping her actually accept that her mum is not here anymore. I also feel like I should be helping her but idk how